In order to abate emission of carbon dioxide gas from automobiles, a reduction in weight of automobile vehicle bodies has been promoted by using high-strength steel sheets. Further, in order also to secure the safety of a passenger, a high-strength steel sheet has been increasingly used for an automobile vehicle body in addition to a soft steel sheet. In order to further promote the reduction in weight of automobile vehicle bodies from now on, the strength of the high-strength steel sheet has to be increased more than conventionally.
In order to use the high-strength steel sheet for an underbody part, for example, burring workability has to be improved in particular. However, when a steel sheet is increased in strength in general, formability decreases, and uniform elongation important for drawing and bulging decreases.
In Non-Patent Document 1, there is disclosed a method in which austenite is made to remain in a steel sheet structure to secure uniform elongation. Further, in Non-Patent Document 2, there is disclosed a method of securing uniform elongation with the same strength by making a metal structure of a steel sheet complex.
Meanwhile, there is also disclosed control of a metal structure that improves local ductility necessary for bending, hole expanding, and burring. Non-Patent Document 3 discloses that controlling inclusions, making a structure uniform, and further decreasing hardness difference between structures are effective for improvement of bendability and hole expandability.
This is a method to improve the hole expandability by making a structure uniform by structure control, but in order to make a structure uniform, a heat treatment from an austenite single phase becomes a basis as disclosed in Non-Patent Document 4.
In order to attain achievement of strength and ductility, Non-Patent Document 4 discloses that a transformation structure is controlled by cooling control, thereby obtaining appropriate fractions of ferrite and bainite. However, all the cases are to improve local deformability relying on the structure control, and desired properties are greatly affected by how the structure is formed.
Meanwhile, as a method of improving a material of a hot-rolled steel sheet, there is disclosed a technique of increasing a reduction amount in continuous hot rolling. This is what is called a technique of making crystal grains fine, in which heavy reduction is performed at as low temperature as possible in an austenite region and non-recrystallized austenite is transformed to ferrite, to achieve making crystal grains of ferrite, which is the main phase of a product, fine.
Non-Patent Document 5 discloses that by this grain refining, increasing strength and increasing toughness are aimed. However, Non-Patent Document 5 pays no attention to the improvement of hole expandability, which is desired to be solved by the present invention, and does not disclose also a means applied to a cold-rolled steel sheet.